Currently, hexagonal strontium and barium ferrites of the magnetoplumbite type (M type) are mainly used as oxide permanent magnet materials and they are manufactured into sintered magnets and bonded magnets.
Very important magnet properties are remanence or residual magnetic flux density (Br) and intrinsic coercivity (HcJ).
The remanence (Br) of a magnet is determined by the density, degree of orientation, and saturation magnetization (4.pi.Is) which is determined by the crystalline structure. Br is thus expressed as Br=4.pi.Is x degree of orientation x density. Strontium and barium ferrites of the M type have a 4.pi.Is value of about 4.65 kG. The density and degree of orientation are limited to about 98% of the theory at maximum even in the case of sintered magnets providing the highest values of density and orientation. Therefore, the Br of these magnets is limited to about 4.46 kG. It is impossible in a substantial sense to obtain a Br of higher than 4.5 kG.
The inventors found in JP-A 9-115715 that by adding a proper amount of La and Zn, for example, to M type ferrite, the 4.pi.Is can be increased about 200 G at maximum. As a result, a Br value of higher than 4.5 kG is obtainable. However, since this was achieved at the sacrifice of anisotropy field (H.sub.A) to be described later, it was difficult to acquire a Br of at least 4.5 kG and a HcJ of at least 3.5 kOe at the same time.
HcJ is proportional to the product (H.sub.A xfc) of an anisotropy field (H.sub.A =2K.sub.1 /Is) and a proportion (fc) of single domain particles. It is noted that K.sub.1 is a crystal magnetic anisotropy constant which is determined by the crystalline structure as is Is. K.sub.1 is equal to 3.3.times.10.sup.6 erg/cm.sup.3 for M type barium ferrite and 3.5.times.10.sup.6 erg/cm.sup.3 for M type strontium ferrite. The M type strontium ferrite is known to have the highest value of K.sub.1 while it is difficult to further increase K.sub.1.
Provided that N is a diamagnetic field coefficient which is determined by the shape of particles, as N becomes greater, a greater diamagnetic field is applied to particles to deteriorate HcJ as seen from the following formula (1). EQU HcJ.varies.(2K.sub.1 /Is-NIs) (1)
In general, as the aspect ratio of particles becomes greater (particles become flattened), N becomes greater and HcJ becomes deteriorated.
On the other hand, if ferrite particles are in single magnetic domain state, maximum HcJ is expectable because the magnetization must be rotated against the anisotropy field in order to reverse the magnetization. In order that ferrite particles become single magnetic domain particles, the size of ferrite particles must be reduced equal to or less than the critical diameter (dc) given by the equation: EQU dc=2(k.multidot.Tc.multidot.K.sub.1 /a).sup.1/2 /Is.sup.2
wherein k is the Boltzmann constant, Tc is a Curie temperature, and a is a distance between iron ions. Since dc is equal to about 1 .mu.m for M type strontium ferrite, a sintered body must be controlled to a crystal grain diameter of 1 .mu.m or less when a sintered magnet is to be fabricated, for example. Although it was difficult in the prior art to realize such fine crystal grains at the same time as the achievement of a high density and high degree of orientation for providing high Br, the inventors proposed a new preparation method in JP-A 6-53064, achieving high properties that were not found in the prior art. Even with this method, however, HcJ is about 4.0 kOe when Br is 4.4 kG. It was thus difficult to acquire a high HcJ of at least 4.5 kOe while maintaining a high Br of at least 4.4 kG.
Also, in order to control the sintered body to a crystal grain diameter of 1 .mu.m or less, the particle size in the shaping stage must preferably be 0.5 .mu.m or less when grain growth in the sintering stage is taken into account. The use of such fine particles generally gives rise to a problem of reduced productivity due to an increase of the shaping time and increased cracks during shaping. It was very difficult to find a good compromise between property and productivity improvements.
On the other hand, it is known in the art that the addition of Al.sub.2 O.sub.3 or Cr.sub.2 O.sub.3 is effective for achieving high HcJ. In this case, a high HcJ of at least 4.5 kOe is obtainable since Al.sup.3+ or Cr.sup.3+ substitutes for Fe.sup.3+ having "upward" spin in the M type structure and thus serves for increasing H.sub.A and suppressing grain growth. However, Is declines and the sintered density tends to decline, resulting in a substantial drop of Br. As a consequence, a composition ensuring a HcJ of 4.5 kOe provides only a Br of about 4.2 kG at maximum.